Vapours' permanent semi-confusion effect causes them to pick a random target square whenever they cast lightning bolt. Usually, nothing they want to hit is in this path...which means the tracer doesn't hit anything and they don't fire. I was actually able to do a 100-turn rest in front of one without it ever shooting. This also causes a lot of spam in debug mode (a couple hundred tracer messages for every one time it actually fires).

This is probably also why ball lightnings would almost never fire back when they had the spell.